With respect to melamine polyphosphate, various types of melamine polyphosphate compounds are disclosed in literatures. For example, JP-B-40-28594 discloses a process which comprises calcining melamine orthophosphate at a temperature of from 180 to 250.degree. C. to obtain a calcined melamine phosphate wherein a part of melamine orthophosphate remains. However, it has been pointed out that this melamine phosphate is a composite of melamine orthophosphate and melamine pyrophosphate and does not have adequate water resistance.
U.S. Pat. No. 3,920,796 discloses that melamine pyrophosphate is formed by calcining melamine orthophosphate at a temperature of from 170 to 325.degree. C.
Further, U.S. Pat. No. 4,950,757 discloses a process for producing melamine pyrophosphate, which comprises reacting pyrophosphoric acid and melamine in an aqueous medium at a temperature of from 0 to 60.degree. C.
JP-A-61-126091 discloses a process for producing melamine condensed phosphate, which comprises subjecting condensed phosphoric acid and melamine to a solid phase reaction substantially in the absence of an aqueous medium at a temperature of from naturally generated temperature to 170.degree. C.
It is known to employ urea as a condensing agent for the production of a polyphosphate. For example, JP-B-53-2170 discloses a process for producing an amide polyphosphate containing amide-type nitrogen, wherein a phosphoric acid source, such as ammonium orthophosphate, orthophosphoric acid, condensed phosphoric acid, phosphoric anhydride, urea phosphate or a mixture thereof, and a nitrogen source, such as melamine, a cyanamide derivative such as dicyan cyanamide, guanidine or guanyl urea, or a mixture thereof, are subjected to a heat condensation reaction in the presence of a condensing agent, such as urea, urea phosphate or a mixture thereof. With respect to the production conditions, it is disclosed that heat condensation is carried out at a molar ratio of urea/phosphoric acid (as H.sub.3 PO.sub.4)/cyanamide derivative=0.8 to 1.5/1/0.05 to 1 in an atmosphere of ammonia gas at a temperature of from 150 to 350.degree. C. for from 10 minutes to 5 hours, preferably from 1 to 4 hours. Example 1 of the same publication discloses that ammonium phosphate, industrial urea and melamine were mixed at a molar ratio of urea/phosphoric acid (as H.sub.3 PO.sub.4)/melamine=1/1/0.5 to obtain a material for calcination, which was subjected to heat condensation at a temperature of from 240 to 260.degree. C. for 2.5 hours to obtain a substantially water-insoluble amide polyphosphate (nitrogen: 33.48 wt %, diphosphorus pentoxide: 44.73 wt %, elution ratio (10 g of a sample was put into 100 ml of water and stirred for 1 hour at a predetermined temperature, whereupon the elution ratio (%) was determined): 1.36%, pH of the 1% solution: 8.26).
JP-B-53-15478 discloses a process for producing an ammonium polyphosphate hardly soluble in water, which comprises calcining crystalline urea phosphate, or ammonium phosphate and urea, as starting material, wherein the starting material is heat-condensed in the presence of melamine or melamine phosphate in an amount of from 5 to 50 wt % as melamine (based on the total amount of the starting material and melamine or melamine phosphate) to obtain a modified ammonium polyphosphate. Example 1 of the same publication discloses that a starting material prepared by mixing ammonium phosphate, industrial urea and melamine at a molar ratio of urea/phosphoric acid (as H.sub.3 PO.sub.4)/melamine=1/1/0.5, was subjected to heat condensation by a rotary indirect heating furnace at a temperature of from 260 to 270.degree. C. for 1.5 hours to obtain a product, then the product was pulverized to a particle size whereby all particles pass through 32 mesh (standard sieve) and then subjected to heat condensation by a rotary indirect heating furnace in an ammonia gas atmosphere at 270.degree. C. for 1.5 hours to obtain a substantially water-insoluble modified ammonium polyphosphate (total nitrogen: 34.52 wt %, ammonia type nitrogen: 8.15 wt %, diphosphorus pentoxide: 44.03 wt %, elution ratio (10 g of a sample was put into 100 ml of water and stirred for 1 hour at a temperature of 25.degree. C., whereupon the elution ratio (%) was determined): 0.31%, pH of the 1% solution: 7.60).
JP-B-55-49004 discloses a process for producing a mixture of a hardly soluble ammonium polyphosphate and melamine, which comprises a solid phase reaction by heating a mixture of melamine phosphate and urea at a temperature of at least 240.degree. C. Example 2 of the same publication discloses a mixture of a hardly soluble ammonium polyphosphate and melamine (phosphorus: 14.5%, water-solubility (10 g of a sample was put into 100 ml of water, followed by shaking at room temperature for 30 minutes, whereupon the elution ratio (%) was determined): 4.8%), which was obtained by using 10 kg of melamine phosphate and 6 kg of urea (1.5 mol of urea per mol of phosphoric acid).
U.S. Pat. No. 4,043,987 discloses a process for producing an ammonium melamine polyphosphate as a salt of a substituted ammonium polyphosphate. Example 1 of the same patent discloses that while stirring 100 g of condensed phosphoric acid, 47 g of urea was added thereto, then 8 g of melamine was added to the mixture, and the entire amount was calcined at 240.degree. C. for 30 minutes to obtain an ammonium melamine polyphosphate as a foam solid (solubility in water at 25.degree. C.: 5.4 g/100 ml, pH of the 10 wt % aqueous slurry at 25.degree. C.: 5.68). The remaining foam solid was pulverized, and the pulverized product was heat-treated by a metal pressure container under a pressure of 4.times.10.sup.5 Pa at a temperature of 170.degree. C. for 13 hours to obtain a pulverized product (carbon: 4%, nitrogen: 18%, phosphorus: 29%, solubility in water at 25.degree. C.: 3.92 g/100 ml, pH of the 10 wt % aqueous slurry at 25.degree. C.: 6.63).
On the other hand, it is well known to utilize a melamine polyphosphate as a flame retardant, and many proposals have been made heretofore. For example, JP-A-53-49054 discloses a polyamide resin composition having improved flame retardancy, which is prepared by adding an inorganic filler and melamine phosphate to a polyamide.
JP-A-61-126091 discloses that condensed melamine phosphate is useful as a flame retardant for a thermoplastic resin such as polyester, polyamide or polyolefin, for a thermosetting resin such as a phenol or urethane epoxy resin, or for a cellulose material.
Further, JP-A-7-316415 discloses a flame retardant polyphenylene ether resin composition, which is prepared by adding melamine phosphate to a polyphenylene ether resin composition.
Each of melamine orthophosphate, melamine pyrophosphate and calcined melamine phosphate disclosed in the above-described prior art has a large water content and no adequate heat stability and thus has a drawback that dissociation of water and melamine is remarkable before the heating temperature reaches 300.degree. C. and a drawback that the content of water-soluble components is high. The above-described condensed melamine phosphate is improved in that dissociation of water and ammonia due to heating at a temperature of 300.degree. C. is little, but still has a drawback that the content of water-soluble components is high. Accordingly, although such melamine orthophosphate and melamine polyphosphates are effective as flame retardants for cellulose materials or resins having low molding temperatures, they can be hardly effectively used as flame retardants for resins having high molding temperatures, with which resin temperatures become 300.degree. C. even temporarily.
Further, the above-mentioned amide polyphosphate prepared by using urea as a condensing agent, the mixture of ammonium polyphosphate and melamine, or the ammonium melamine polyphosphate, is improved in that dissociation of water by heating at a temperature of 300.degree. C. is little, but undergoes thermal decomposition and dissociation of ammonia. Further, most of them have a drawback that the content of water-soluble components is high. Accordingly, they may be effective as flame retardants for cellulose materials or resins having low molding temperatures, but can be hardly effectively used as flame retardants for resins having high molding temperatures, with which the resin temperatures become 300.degree. C. even temporarily.